The invention relates to a gasoline engine with an exhaust catalyst for reducing the emission of pollutants.
Operation of gasoline engines with a lean air/fuel mixture in order to reduce fuel consumption is known. In this manner, fuel savings of up to 10% can be achieved in the lower partial load range. As a result of the oxygen surplus in the exhaust during lean operation, however, the three-way catalyst can no longer maintain its function of reducing the oxides of nitrogen contained in the exhaust by residual hydrocarbons. In Denox catalysts the oxygen surplus can result in poisoning of the active catalyst surface, considerably reducing the conversion rate.
German patent document DE-OS 38 07 907 discloses an arrangement in which an oxygen absorption device is located in the exhaust line of an internal combustion engine upstream from the three-way catalyst to reduce the oxygen component in the exhaust, adsorbing or desorbing oxygen from the exhaust line as a function of the partial pressure. This arrangement affects the composition of the exhaust gas because of its buffer action, with the oxygen component contained in an oxygen rich exhaust being reduced. It is not feasible, however, to use this device to regulate the oxygen content in the exhaust directly as a function of the operation of the internal combustion engine. It is impossible to operate the gasoline engine in different ranges of the air/fuel mixture without considerably adversely affecting the functional ability of the catalyst.
A gasoline engine with an NO.sub.x absorption device is disclosed in European patent document EP-0 581 279 A2, in which the oxides of nitrogen are adsorbed in a lean operating range following oxidation on a platinum surface. The oxygen content of the exhaust is reduced by this oxidation. However, these oxides of nitrogen are returned to the exhaust later, so that the air/fuel mixture ratio must be increased to the point where the percentage of unburned hydrocarbons is sufficient to reduce them.
One object of the present invention is to provide an improved gasoline engine with a catalyst, that reduces fuel consumption without adversely affecting the release of pollutants.
This object is achieved according to the invention by providing an oxygen removal device that can be activated or deactivated by a control device, and which not only buffers the oxygen from the exhaust on an intermediate basis but actually removes it from the exhaust line. In this manner, it is possible to regulate exhaust composition in different operating modes of a gasoline engine. Under full load, the gasoline engine can be operated in the conventional manner, with the air/fuel ratio being regulated to a value in the range .lambda.=1 and the oxygen removal device being deactivated. In the partial load range the gasoline engine is switched to a lean air/fuel ratio, and the oxygen removal device is activated, thus achieving a considerable fuel saving. In the exhaust, a composition corresponding to operation at .lambda.=1 is achieved by oxygen removal, so that no deterioration of the pollutant emission is produced (in contrast to a conventional gasoline engine with a three-way catalyst).
When a Denox catalyst is used, it is possible to operate the gasoline engine constantly with a lean air/fuel ratio. If poisoning phenomena occurs because of an oxygen surplus in the catalyst, the oxygen removal device can be activated briefly and the catalyst surface reduced again as a result.